1. Field of the Invention
The present invention relates to a particle sampling apparatus and its operating method for semiconductor device manufacturing. More particularly, the present invention relates to a particle sampling apparatus for sampling particles directly from the processing chamber of a vacuum processor and its operating method.
2. Description of the Related Art
Semiconductor device manufacturing processes require very clean processing environments. Several manufacturing processes, including Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), dry etch, sputtering, and ion injection, require a vacuum state during processing. The above processes are subject to various failures depending on the processing equipment and the corresponding processing gas used. A great number of failures of semiconductor devices are caused by particles generated in a processing chamber. In order to determine how to minimize and contain these damaging particles, it is necessary to analyze and quantify the distribution of generated particles.
Conventionally, the particles and defects present on wafers are analyzed after the wafers are processed and removed from the chamber. However, it is often impossible to determine the exact cause of the damaging particles because the particles can not be observed during the sequence of events carried out in the chamber during a process.
An impactor is one conventional device that is capable of directly sampling particles from a processing chamber. However, a drawback of the impactor is that it is designed to sample such particles only while a high pressure process is being performed in the process chamber.
Referring to FIG. 1, an impactor or particle sampler 10, collects particles by passing a gas released directly from inside the processing chamber through the sampler from the left inlet to the right outlet as designated by the arrows in FIG. 1. Particle collection wafers are placed on stages 14 and 15 oriented perpendicular to the direction of gas flow. For example, the particle sampler 10 in FIG. 1 has two stages, a first stage 14 and a second stage 15. A first nozzle 12 and a second nozzle 13 are formed facing stages 14 and 15, respectively; and nozzles 12 and 13 have different diameters.
When a pressure gradient is applied from the left inlet to the right outlet of the particle sampler 10, sample air containing particles passes through the first nozzle 12, and collides with the collecting wafer on the first stage 14 by inertia so that the particles are collected according to the speed and the mass of the particles. Then, the sample gas that collided with the first stage 14 passes through the second nozzle 13 having a smaller diameter than that of the first nozzle 12 so that the gas and particles are accelerated. The accelerated particles collide with the collecting wafer on the second stage 15. When the speed of the sample gas is sufficiently fast, very small particles will collide with, and can be collected on, the collecting wafer.
Conventionally, the impactor particle sampler is used for the collection of particles when the processing chamber is under high pressure. However, it cannot be used if the sampled gas is poisonous. If the processing gas in the processing chamber is poisonous, it must be replaced with a safer gas, such as nitrogen gas, before particle sampling is performed.
During vacuum processing, on the other hand, particle sampling can only be carried out using a vacuum pump to establish a pressure difference between the processing chamber and a pumping line downstream of the particle sampler. Particle sampling is accomplished using equipment with a sampling port that can be connected to the processing chamber, and a cut-off valve, a particle sampler, and another cut-off valve, installed in sequential order on a line from the sampling port. The sample gas is discharged through a discharge line by the vacuum pump. Then, while a vacuum process is performed in the processing chamber, the cut-off valves are opened for a certain time and some contents from the processing chamber are passed through the particle sampler where the particles are collected. The cut-off valves are then closed; then the particle sampler is disconnected from the processing chamber. Next, the collecting wafers are dismounted from the stages and particles on the collecting wafers are then analyzed.
If a vacuum process in the processing chamber is performed at a high enough vacuum, i.e., a low enough pressure, the vacuum pump of the particle collecting system can not maintain the proper pressure gradient. Then gas in the particle sampler may move in the opposite direction, carrying particles into the processing chamber. This condition is called back-flow, and it is undesirable because it increases the likelihood of damage to the semiconductor device in the processing chamber.
In addition, the particle sampler containing the collected particles must be completely purged before it is ready for subsequent use. After purging, the particle sampler must be reconnected to the processing chamber. However, the reconnecting task can again contaminate the particle sampler so that extra particles are introduced into the sampler. This can lead to a failure of the particle sampler to provide an accurate sample for analysis.
Thus there is a need for a particle sampling apparatus that can directly sample particles from a process chamber reliably, repeatedly and efficiently, whether the chamber is in a high pressure state or an extremely low pressure state. At high pressure, leaks must be prevented. At low pressure back-flow must be prevented. Purging must be leak proof and should not require disconnecting and reconnecting the apparatus to the chamber, to prevent contamination of the sampler after purging.